Definitely Ladybug and Chat Noir. They are the central characters, and a Kwami swap for them means a whole new set of challenges and adventures. For example, how they interact with each other when they have different powers and how they adapt to the changes. Also, Rena Rouge could be an interesting character to focus on in Kwami swap fanfiction. Maybe she gets involved in helping the heroes deal with the consequences of the swap, or perhaps her own Kwami gets swapped in some way, adding another layer of complexity to the story.
Marinette and Adrien are still very popular in Kwami Swap Fanfiction. Since they are the main characters in the original series, seeing them with swapped Kwami powers is quite exciting. For example, Marinette as the Cat Noir - like character brings a whole new charm. Also, Alya sometimes becomes a popular character in these fictions as she might get involved in the Kwami swap scenarios in interesting ways.
Marinette and Adrien are definitely the most popular. Their relationship is a central part of the Miraculous Ladybug story, and in a college setting, there are so many new dynamics to explore. For example, how they deal with newfound independence away from home while still having their superhero responsibilities.
In a Kwami swap fanfiction, a possible plot is that the swapped powers start to have unforeseen side effects on the holders' daily lives. Ladybug, for instance, might find it difficult to balance her normal life with the new Kwami - influenced powers. Chat Noir could be in a similar situation. As they struggle to deal with these changes, they also have to figure out how to reverse the Kwami swap. Meanwhile, the city is in more danger than ever as they are not at their full potential, and this could lead to some intense and exciting battles and rescue scenes.
If the kwami swap involves a kwami with a mischievous nature, Lady Noir's personality could become more playful. She might start cracking more jokes during battles, much like Chat Noir usually does. This would change her overall demeanor and how she interacts with both the villains and Chat Noir. It could also make their partnership more light - hearted or, if Chat Noir isn't used to it, a bit more chaotic as they adjust to this new side of Lady Noir.
The kwamis themselves are also very much affected. Tikki and Plagg, for example. They have to adapt to new holders. They might have different ways of communicating and guiding their new heroes. Their own little 'kwami - world' is disrupted too. And other characters like Alya and Nino might also notice the changes in Marinette and Adrien and get involved in the story in different ways.
One popular theme is the exploration of the Kwamis' personalities. For example, Tikki's positive and guiding nature can be delved into more deeply. Another theme is the relationship between the Kwamis and their holders. How they interact, communicate, and support each other during battles and in daily life. Also, some fanfictions focus on the origin of the Kwamis, creating backstories that are not fully explored in the original series. This could include where they came from before they were associated with the Miraculous jewels and how they came to be part of the whole Miraculous system.
Ladybug and Chat Noir are obvious choices. Their dynamic is always interesting to explore further. You can show how they grow and develop as heroes. Also, the Puppeteer herself is a great character to focus on. Her motives and powers make her a complex character that can be delved into deeper in fanfiction.
Tikki is really interesting. She's the Kwami of Creation and is always positive and full of wisdom. There are so many aspects to explore, like how she encourages Marinette in difficult situations. Plagg is also great. His mischievous nature as the Kwami of Destruction provides a lot of comical and interesting storylines. Another one is Wayzz. His calm and knowledgeable demeanor makes him a great character to write about, especially when it comes to guiding his holder about the powers and history of the Miraculous.
Hawk Moth is another character that shows up a lot in fanfiction. Writers like to explore his motives more deeply, or come up with scenarios where he has a change of heart or forms an unexpected alliance. His complex character makes him an interesting subject for fanfiction writers to play around with.
Aside from Ladybug and Chat Noir, Hawk Moth is quite popular in fanfiction. He is the main villain, and many fanfiction writers like to explore his backstory and motives further. There are also some side characters like Alya, Marinette's best friend, who is popular as she often gets involved in the superhero adventures in an indirect way. And of course, Adrien, who is Chat Noir, is a fan favorite due to his charming personality both as a superhero and as a civilian.